Distinction between potentially treatable immune motor neuropathies and other disorders of lower motor neurons depends partly on electrodiagnostic evaluations and serum antibody testing. Nerve conduction testing showing motor conduction block at non-entrapment sites along the length of nerves, especially in distal regions, is widely recognized as a marker of the treatable syndrome, multifocal motor neuropathy (MMN). In addition, serum IgM binding to GM1 ganglioside is common in, and has specificity for, multifocal and other immune motor neuropathies. Testing for IgM anti-GM1 antibodies is especially helpful when electrodiagnostic findings of demyelination are equivocal or negative.
Electrophysiological testing, and testing for IgM anti-GM1 antibodies are not however dispositive of neuropathy in a given subject. For example, in most studies the sensitivity of IgM anti-GM1 antibody testing for MMN ranges from only 30% to 50%. Testing for additional antibodies, including IgM binding to GM2 and GalNAc-GD1a gangliosides, can add some sensitivity, but many patients with immune motor neuropathies have no identified associated serum autoantibody. Accordingly, a need remains for improved methods of diagnosing neuropathy, particularly motor neuropathy, as the cause of clinical symptoms in a subject. Because immune motor neuropathies are potentially treatable, a correct differential diagnosis against certain less treatable alternative diagnoses is especially important. A need therefore remains for improved methods for differentially diagnosing such motor neuropathies, thus to promote more timely treatment.